The present invention relates to the vaporization of cryogenic liquids. In more detail, the present invention relates to mobile vaporizers and methods for vaporizing LNG in sufficient volume and pressure to enable the use of the vaporized natural gas as a fuel source or in applications in which large quantities of vaporized LNG/compressed natural gas (CNG) are needed, for instance, for internal combustion engines or gas turbines operating at high horsepower, for electrical power generation during periods of peak demand, to displace other fuels such as oil or coal as fuel for power generation, and for well servicing applications utilizing CNG.
Recent increases in the cost of diesel fuel and changes in regulations governing the emissions from diesel engines have motivated the operators of diesel fleets to investigate the use of alternative fuels. The price advantage and availability of natural gas make it one of several alternative fuels currently under consideration as a substitute or at least a supplement to diesel fuel (see, for instance, M. Howe, “Mining Majors Ponder Switch to LNG in Bid to Cut Costs,” http://designbuildsource.com.au/mining-majors-ponder-switch-to-lng-a-bid-to-cut-costs (Apr. 13, 2013)). In addition to the 30-40% price advantage, the ease of transport, and the cleaner emissions, natural gas has the advantage of performing in a manner similar to diesel in many applications and is therefore a focus of major diesel engine manufacturers. For example, Caterpillar is already offering natural gas-powered products such as mining trucks and locomotives, as well as bi-fuel conversion kits for existing Caterpillar diesel engines. Further, after-market component manufacturers such as American Power Group, Inc. are delivering bi-fuel conversion kits allowing various makes of existing engines to run on a mix of diesel and natural gas. For these same reasons, and because of the need for on-site natural gas, sometimes in remote locations away from the existing pipeline grid, natural gas is of interest as a fuel source and for well service applications in oil and gas production and as an alternative fuel source in the power generation industry. Baker-Hughes, Halliburton and other oilfield services players have recently announced initiatives to convert diesel-fueled drilling rigs and frac fleets to natural gas, and one Canadian natural gas producer reported savings of approximately $11 million in fuel costs in 2011 by using natural gas as a fuel source for its rigs.
Despite the cost savings and significantly decreased emissions, there are barriers to the widespread use of natural gas in such applications as the oilfield. Although natural gas can be transported to locations remote from the existing pipeline grid and stored in large quantity as liquefied natural gas (LNG), the LNG must be vaporized for use at the remote site. Vaporization of cryogenic liquids, and specifically, LNG vaporization is known in the art. By way of example, mobile LNG vaporizers are operated and/or sold by Eleet Cryogenics, Inc. (Bolivar, Ohio), Chart Industries, Inc. (Garfield Heights, Ohio), Prometheus Energy (Houston, Tex.), INOXCVA (Houston, Tex.) and others. In their most basic form, LNG vaporizers consist of ambient air heat exchangers equipped with the necessary valves and piping to receive the LNG from a reservoir and output the gas to the on-site equipment. Other types of vaporizers produce the heat required to vaporize LNG with electrically-powered heaters (one of the Chart Industries vaporizers utilizes an electric heater, for instance) or a fired heat source such as a burner (for instance, a Prometheus Energy vaporizer that circulates heated water through a heat exchanger for vaporizing the LNG).
However, the technology of the LNG vaporizers currently in use is an obstacle that prevents widεspread use of LNG as a fuel source in such applications as the oilfield in which large volumes of natural gas are required to meet on-site energy needs. So far as is known, for instance, LNG mobile ambient vaporizers currently in use in the oilfield produce about 500-3000 standard cubic feet/min. of natural gas output at about 60 psi. The heat exchangers of such vaporizers require a large surface area to extract sufficient ambient heat for vaporizing LNG, thus requiring that the vaporizer have a large footprint, and the efficiency of such vaporizers is dependent on ambient air temperature. Further, ice build-up on the heat exchanger surface area imposes limits on the number of hours the vaporizer can operate and overall vaporizing efficiency. One way to overcome this ice build-up problem is to switch between multiple vaporizers, but using multiple vaporizers further increases the overall footprint of the vaporizer and limits the mobility of the vaporizer such that moving the vaporizer from one site to another may require multiple truckloads and over-the-road weight and height special permits. Electric type vaporizers such as those provided by Chart Industries, Inc. and INOXCVA are advertised as being capable of approximately 500 to 1000 SCFM at pressures of about 60 psi. This type of vaporizer is typically an integral component of an LNG transport tanker and requires an electrical source on-site, and such power sources are not always available at some sites such as hydraulic fracking locations. Further, multiple vaporizers of this type are needed on locations requiring high LNG flow such that overall footprint is large and mobility is compromised by the need for multiple truck loads to move the vaporizer(s) from site to site. By way of actual example, the above-described Prometheus Energy vaporizer is said to produce flow rates of about 250,000 scfh at approximately 60 psi. However, such vaporizers require a fired fuel source, typically a burner/boiler, that must be located at least 50-100 feet away from the LNG source, which is a limiting factor in some tight locations and could present safety concerns.
The modest pressure and rate output of these known/existing vaporizers, and other limiting factors, result in part from the need to move the vaporizer from one site to another, which imposes size and weight limitations on the vaporizer, and the need to limit the on-site footprint while maximizing the flexibility of locating the vaporizer in tight locations (the latter being of particular concern where a fired source is used as a source of heat), impose restrictions on the overall utility of known/existing vaporizers. So far as is known, no currently available LNG vaporizer (other than those that are part of a permanent installation and that require substantial capital outlay, for instance, at a natural gas-fired electric (or dual fuel) power generation plant) capable of overcoming these limitations and restrictions that outputs natural gas at high enough pressure and in sufficient volume for use as a fuel source for such high power applications as a fleet of ten or more on-site frac pumpers operating at the high horsepower required to pump fluids through a horizontal wellbore at the pressures needed for fracturing a hydrocarbon formation. To further illustrate, in recent years, manufacturers of bi-fuel kits that allow existing diesel engines that run on nearly 70/30 mix of diesel and natural gas, and new engine manufacturers, are developing new natural gas and bi-fuel engines that are even lighter, smaller in size and capable of producing higher horsepower than currently available.
Despite on-going engine development efforts, as far as is known, no mobile LNG vaporizers are available that are not encumbered by limitations of size, weight, mobility, flow capacity, and particularly the ability to output CNG at the high pressures sufficient to power even a small fleet of frac pumpers running on a 70/30 mix of diesel and LNG. For these same reasons, so far as is known, existing vaporizers are incapable of outputting gas at pressures sufficient to allow the mobile LNG vaporizer and the LNG storage tanks (mobile or stationary) to be located at a distance of several hundred feet, or even several miles, away from the site where the natural gas flow is needed. A mobile vaporizer capable of delivering compressed natural gas at pressures higher than existing vaporizers can also provide advantages not possible with existing mobile vaporizers in addition to allowing the vaporizer to be located a safe distance away from the site where natural gas is needed.
A significant increase in maximum vaporizing capacity (up to 540,000 scfh) and output pressure could enable an LNG vaporizer to be used for new applications including planned natural gas supply interruptions and even as a supply source for several miles of temporary pipeline. In the oilfield, mobile vaporizers that could produce sufficient pressure and volume to power a fleet of frac pumpers having 2000-2500 horsepower diesel engines equipped with bi-fuel kits, allowing those engines to run on nearly a 30/70 mix of diesel and LNG, would enable the above-described advantages of the small footprint needed in tight remote locations and the need to locate the LNG vaporizer and LNG source remotely from the frac fleet. Again, the oilfield provides just one example of an application in need of vaporizers with higher output than currently available; there are many other applications including on-demand and off the grid power generation, temporary heating, and offshore oil rigs.
It is therefore an object of the present invention to provide an LNG vaporizer that outputs sufficient volumes of vaporized natural gas at sufficient pressure to provide fuel for, for instance, a fleet of on-site frac pumpers running on natural gas or a mix of diesel and natural gas.
It is another object of the present invention to provide an LNG vaporizer small and light enough to be moved from location to location that outputs high volumes of vaporized natural gas at high pressure; specifically, it is contemplated that the LNG vaporizer of the present invention mounts on a trailer, tractor, or skid weighing less than about 60,000 pounds so that the vaporizer is road legal and load permits are not required to move the vaporizer on public roads.
As noted above, another problem with existing LNG vaporizers is the relatively low pressure, usually about 40 to 60 psi, of the natural gas output from the vaporizer. Most known LNG vaporizers output vaporized natural gas at about 40 psi (because 40 psi can be achieved by expansion from heat), but in an application in which large volumes and/or high pressure LNG is required (such as the fleet of frac pumpers described above and/or in applications in which the LNG supply and vaporizer must be located remotely from the internal combustion engine for safety or other reasons), pressures in the 500 to 2500 psi range, or higher, may be required. It is therefore also an object of the present invention to provide an LNG vaporizer having the flexibility of being capable of outputting natural gas at low and high pressures depending upon the application/location and demand.
Other objects, and the many advantages of the present invention, will be made clear to those skilled in the art in the following detailed description of the preferred embodiment(s) of the invention and the drawing(s) appended hereto. Those skilled in the art will recognize, however, that the embodiment(s) of the present invention that are described herein are only examples of specific embodiment(s), set out for the purpose of describing the making and using of the present invention, and that the embodiment(s) shown and/or described herein are not the only embodiment(s) of an apparatus and/or method constructed and/or performed in accordance with the teachings of the present invention. Further, although described herein as having particular application to certain oilfield operations, as noted above, those skilled in the art who have the benefit of this disclosure will recognize that the present invention may be utilized to advantage in many applications (gas turbine and pipeline commissioning and testing, peak electric generation, emergency or planned natural gas supply interruptions, well servicing and completion operations, and many others) in which large quantities of natural gas are being used as a fuel source or for other purposes where high pressure natural gas is required, the present invention being described with reference to the oilfield for the purpose of exemplifying the invention, and not with the intention of limiting its scope.